System, method and fiber-optic transceiver module for bandwidth efficient distortion-tolerant transmissions for high-bit rate fiber optic communications

ABSTRACT

According to one embodiment of the invention, fiber optic communications method is described. The method comprises a first operation of dynamically identifying frequencies at which spectral nulls occur in a signal received via an optical fiber, and thereafter, segregating communications over the optical fiber into a set of inter-null bands defined by the spectral nulls.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromU.S. Provisional Patent Application No. 61/588,057 filed Jan. 18, 2012,the entire contents of which are incorporated herein by reference.

FIELD

Embodiments of the invention generally relate to optical data linksincluding fiber optic transmitters, receivers and transceivers.Particularly, embodiments of the invention relate to circuitry, deviceand method for identifying channel nulls and transmitting information inthe frequency bands between these channel nulls.

GENERAL BACKGROUND

In order to lower the cost of communications, it has become desirable toincrease data rate and the number of communication channels availablefor such communications. This is particularly true in fiber opticcommunication systems.

In fiber optic communication systems, wavelength division multiplexing(WDM) has been used over the same fiber optic communication link so thatmultiple channels of communication may be established over the sameoptical link. The multiple channels of communication are established atdifferent center wavelengths of light. However, at high datatransmission rates, fiber dispersion distorts the optical signal, whichadversely affects reliability for correctly recovering data over longdistance data transmissions.

In particular, for direct detect systems, fiber dispersion manifestsitself as channel nulls in the power spectral density (PSD) of thetransmitted signal. These “channel nulls” are substantial reductions inenergy that may distort the perceived content within the signal. Asoptical communication links increase in distance, more channel nulls areexperienced. In other words, optical signals experience more distortionas they travel over a longer optical communication link. For instance,2-3 channel nulls may be experienced when the optical communication linkis approximately two-hundred kilometers (200 Km) in length. However,approximately seven (7) channel nulls may be experienced when theoptical communication link is approximately six-hundred kilometers (600Km).

Hence, removal or mitigation of distortion experienced on theselong-haul (single-mode) optical communication links would improve theoperations of a network, especially for upcoming technologies where theoptical fiber communication links are operating at bit rates greatlyexceeding 10 gigabits per second such as 100 gigabits per second (100Gbps) or more.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of embodiments of the invention will becomeapparent from the following detailed description in which:

FIG. 1 is a general, exemplary embodiment of a fiber optic communicationsystem.

FIG. 2A is a first exemplary embodiment of the power spectral density(PSD) for signaling propagating over an optical fiber medium of a firstlength.

FIG. 2B is an exemplary embodiment of the fiber response for signalingpropagating over an optical fiber medium as illustrated in FIG. 2A.

FIG. 2C is a second exemplary embodiment of the power spectral density(PSD) for signaling propagating over an optical fiber medium of a secondlength.

FIG. 2D is an exemplary embodiment of the fiber response for signalingpropagating over an optical fiber medium as illustrated in FIG. 2C.

FIG. 3 is a detailed, exemplary embodiment of the fiber opticcommunication system.

FIG. 4 illustrates a perspective embodiment of a first system of FIG. 1.

FIG. 5 is an exemplary block diagram of a fiber-optic module of FIG. 1.

FIGS. 6A and 6B are illustrative embodiments of inter-null bandmultiplexing operations.

FIG. 7 illustrates an exemplary flowchart illustrating operations of theINBM-based transmitter is shown.

FIG. 8 illustrates an exemplary embodiment of transmitter logic forsupporting inter-null band data transmissions.

FIG. 9 illustrates an exemplary embodiment of the flowchart highlightingoperations of the transmitter logic of FIG. 8.

FIG. 10 illustrates an exemplary embodiment of receiver logic forsupporting inter-null band data transmissions.

FIG. 11 illustrates an exemplary embodiment of the flowcharthighlighting operations of the receiver logic of FIG. 10.

FIG. 12 illustrates an exemplary embodiment of transmitter and receiverlogic for supporting inter-null frequency using optical carriers.

DETAILED DESCRIPTION

Embodiments of the invention set forth in the following detaileddescription generally relate to a method, device, software, and systemfor mitigating distortion that affect light pulses as they propagateover an optical fiber medium by avoiding data transmissions withinchannel nulls.

The embodiments of the invention are directed to circuitry that isdesigned to estimate null channel locations based on analysis of theoptical fiber medium (e.g., distortion coefficient value and length ofthe optical fiber medium) or analysis of test signals propagated overthe medium. Thereafter, data is organized for transmission within theinter-null bands, namely the frequency bands between the channel nulls.

Herein, certain terminology is used to describe features for embodimentsof the invention. For example, an “optical system” generally refers to adevice that includes logic adapted to transmit and/or receive signalingover an optical fiber medium. The system may further be adapted toprocess information within such optical signaling.

It is contemplated that the optical system may include hardware logic,including one or more of the following: (i) processing circuitry; (ii)one or more lasers for generating light pulses transmitted over theoptical fiber medium; (iii) one or more optical detectors; and (iv) anon-transitory computer-readable storage media (e.g., a programmablecircuit; a semiconductor memory such as a volatile memory such as randomaccess memory “RAM,” or non-volatile memory such as read-only memory,power-backed RAM, flash memory, phase-change memory or the like; a harddisk drive; an optical disc drive; or any connector for receiving aportable memory device such as a Universal Serial Bus “USB” flashdrive).

Additionally, the term “logic” is generally defined as hardware and/orsoftware. As hardware, logic may include processing circuitry (e.g., acontroller, a microprocessor, a digital signal processor, a programmablegate array, an application specific integrated circuit, etc.),semiconductor memory, combinatorial logic, or the like. As software,logic may be one or more software modules, such as executable code inthe form of an executable application, an application programminginterface (API), a subroutine, a function, a procedure, an objectmethod/implementation, an applet, a servlet, a routine, a source code,an object code, a shared library/dynamic load library, or one or moreinstructions. These software modules may be stored in any type ofnon-transitory storage medium (described above) or transitorycomputer-readable transmission media (e.g., electrical, optical,acoustical or other form of propagated signals such as carrier waves,infrared signals, digital signals).

The term “link” is broadly defined as a logical or physicalcommunication path such as, for instance, one or more optical fibers.The term “band” constitutes a range of frequencies with a centerfrequency.

Lastly, the terms “or” and “and/or” as used herein are to be interpretedas an inclusive or meaning any one or any combination. Therefore, “A, Bor C” and “A, B and/or C” mean “any of the following: A; B; C; A and B;A and C; B and C; A, B and C.” An exception to this definition willoccur only when a combination of elements, functions, operations or actsare in some way inherently mutually exclusive.

Referring now to FIG. 1, a general embodiment of a fiber opticcommunication system 100 is shown. In the fiber optic communicationsystem 100, a first system 110 is optically coupled to a second system120 by means of optical communication channels 130 ₁-130 _(N) (whereN≧1). Each optical communication channel 130 ₁-130 _(N) may bebi-directional, and if so, includes a first fiber optic communicationlink 132 and a second fiber optic communication link 134. If onlyunidirectional communications are desired, one of the first or secondfiber optic communication links 132 or 134 can suffice for thecommunication channel depending upon the desired direction of the datatransfer. Fiber optic communication link 132 and 134 may be implementedwithin a single fiber optic cable or within separate cables.

Wavelength division multiplexing (WDM) may be used over each of thefiber optic communication links 132 and/or 134 to accommodate multiplechannels of communications over the fiber optic cable. Bi-directionalcommunication may also be provided over one fiber optic communicationlink 132 or 134 by using different wavelengths of light within the samefiber optic cable.

First system 110 comprises one or more fiber-optic transceiver modules140 ₁-140 _(N). Similarly, second system 120 includes one or morefiber-optic transceiver modules 150 ₁-150 _(N). Each of the fiber-optictransceiver modules 140 ₁-140 _(N) and 150 ₁-150 _(N) include atransmitter (TX) 160 and/or receiver (RX) 170 in order to providebi-directional communications. If unidirectional communication isdesirable, a transmitter TX 160 may be placed within first system 110while a receiver RX 170 would be placed at second system 120 instead ofdeployment of a transceiver at both systems 110 and 120.

Photons or light signals (e.g., data) are generated by transmitter TX160 in first system 110; transmitted through a fiber optic cableassociated with link 132; and received by receiver RX 170 of secondsystem 120. On the other hand, transmitter TX 162 of second system 120can generate photons or light signals (e.g., data) and transmit themthrough a fiber optic cable, via link 134, which can then be received byreceiver RX 172 of first system 110. Thus, communication system 100 canutilize photons or light signals to bi-directionally communicate datathrough the fiber optic cable(s) and its respective links between firstsystem 110 and second system 120.

Referring to FIGS. 2A and 2B, detailed embodiments of the power spectraldensity (PSD) of illustrative signaling propagating over optical fibermedium, such as first fiber optic communication link 132 of FIG. 1, isshown. As shown in FIG. 2A, for link 132 having a two-hundred kilometer(z=200 Km) length with a prescribed distortion coefficient of the fiber(β₂), null channels (A₁ 210, A₂ 220) for PSD 200 are detected around 4.3Gigahertz (GHz) and 7.5 GHz frequency ranges. Channel nulls (A₁, A₂) 210and 220 coincide with in-phase components (B₁ 230, B₂ 240) of fiberresponse 250 as illustrated in FIG. 2B.

The equation for estimating channel nulls is dependent on length (z) andthe distortion (dispersion) coefficient (β₂) and may be represented byequation (1) as shown below (where “k” is merely a constantcorresponding to the particular ordering of the channel null (e.g., k=0for first channel null, k=1 for the second channel null, etc.):

$\begin{matrix}{f = {\frac{1}{2\pi}\sqrt{\frac{\left( {{2k} + 1} \right)\pi}{\beta_{2}z}}}} & (1)\end{matrix}$

Similarly, as shown in FIG. 2C, for link 132 having a six-hundredkilometer (600 Km) length with a prescribed distortion coefficient ofthe fiber (β₂), channel nulls 260 (A₁-A₇) for PSD 270 are detectedaround 2.5 GHz, 4.3 GHz, 5.6 GHz, 6.6 GHz, 7.5 GHz, 8.3 GHz and 9.1 GHzfrequency ranges, respectively. Channel nulls (A₁-A₇) 260 coincide within-phase components (B₁-B₇) 280 of fiber response 290 as illustrated inFIG. 2D.

Referring now to FIG. 3, a more detailed embodiment of fiber opticcommunication system 100 is shown. Herein, fiber optic communicationsystem 100 is a long haul fiber optic communications channel with one ormore repeaters 300 ₁-300 _(M) (M≧1) between the ends of thecommunications channel. While such communications involve aunidirectional channel from transmitter 160 to targeted receiver 170, ofcourse, it is contemplated that fiber optic communication system 100 canbe readily expanded to support bi-directional communications.

From first transmitter 160 to a first repeater 300 ₁ is a first fiberoptic cable 310. Between repeaters 300 ₁-300 _(M) are fiber optic cables320 ₁-320 _(M-1). Between the last repeater 300 _(M) and the lastreceiver 170 is another fiber optic cable 330. The lengths of the fiberoptic cable 310, fiber optic cables 320 ₁-320 _(M-1), and fiber opticcable 330 are typically as large as possible in order to reduce thenumber of repeaters 300 ₁-300 _(M).

Each repeater 300 ₁-300 _(M) includes at least one receiver electricallycoupled to a transmitter. In one embodiment, however, each repeater 300₁-300 _(M) may include one or more transceivers.

FIG. 4 illustrates a perspective embodiment of first system 110. Asshown, first system 110 comprises a plurality of fiber-optic subsystems400 (e.g., optical routers, bridges, or any optical transmitting and/orreceiving components) that are positioned in close proximity to eachother. For instance, as an illustrative example, a number of fiber-opticsubsystems 400 may be positioned on a rack 410 and coupled to fiberoptic cables 420 that interconnect first system 110 with other systemsin different geographic areas. Each of the fiber-optic subsystems 400comprises at least one fiber-optic module 500 operating as either (i) atransceiver (e.g., transceiver module 140 ₁), (ii) a transmitter, or(iii) a receiver.

Referring now to FIG. 5, an exemplary of a fiber-optic module 500 isillustrated. As shown, fiber-optic module 500 includes an integratedcircuit 510 mounted therein to a printed circuit board 520 thatincorporates embodiments of the invention (e.g., at portions oftransmitter logic 800 of FIG. 8 and/or receiver logic 1000 of FIG. 10).As discussed previously, integrated circuit 510 may be one or moreapplication specific integrated circuits (ASICs) to support theelectronics of transmitter and/or receiver. Fiber-optic module 500further includes a light transmitter 530 (e.g., an electrical-to-optical“EO” converter) and/or a light receiver 540 (e.g., anoptical-to-electrical “OE” converter). Fiber-optic module 500 may becompatible with the 10 gigabit per second (10 GPS) small form-factorpluggable multi-source agreement (XFP), 100 GPS form-factors or otherproprietary or standard packages.

Printed circuit board 520 includes top and bottom pads (top pads 522illustrated) to form an edge connector 560 to couple to a socket of ahost printed circuit board (not shown). A housing 570 is positionedaround printed circuit board 520 to protect and shield integratedcircuit 510. A front fiber optic plug receptacle 580 is provided withopenings 582 to interface with one or more fiber optic cables and theirplugs. A mechanical latch/release mechanism 590 may be provided as partof the fiber-optic module 500. While fiber-optic module 500 has beendescribed has having both light transmission and light receptioncapability, it may be a fiber optic transmitter module with lighttransmission only or a fiber optic receiver module with light receptiononly.

Referring now to FIGS. 6A and 6B, illustrative embodiments of inter-nullband multiplexing operations are shown. Herein, frequency sub-bands 600and 650 between channel nulls 610 and 660 are available for datatransmission. As illustrated in FIG. 6A, for example, the location andbandwidth of frequency sub-bands 600 may be established as the frequencyranges between neighboring channel nulls 610 as computed from equation(1) as set forth above. As the RF carrier frequency increases, thebandwidth associated with the frequency sub-bands decreases. Forinstance, a frequency sub-band up to first channel null 610 (leftmostchannel null in FIG. 6A) is greater than the frequency sub-band from thefirst channel null to a second channel null.

In order to mitigate non-linear signal distortion, which is caused byinteractions between sub-phase modulation and dispersion effects, datais transmitted in frequency sub-bands 600 only (i.e. excluding datatransmissions at frequencies associated with the channel nulls). Thistransmission technique, referred to as “inter-null band multiplexing” or“INBM”, mitigates dispersion so that the optical fiber behaves as if itis a dispersion-free medium. In other words, non-linear distortion ofthe optical signal can be greatly mitigated, even for long-haultransmissions (e.g., data transmissions over an optical fiber cablegreater than 1000 km in length).

INBM also provides additional benefits. For instance, INBM also enablesa variety of well-established modulation techniques in wireless andsatellite communications to be used for transmissions in each frequencysub-band over the optical fiber. Examples of these types of modulationtechniques include, but are not limited or restricted to OrthogonalFrequency Division Multiplexing (OFDM), multi-level RF modulation (e.g.,Quadrature Phase Shift Keying “QPSK”, Quadrature Amplitude Modulation“QAM”, etc.) or Trellis-coded modulation.

Additionally, INBM is a non-coherent technique, namely that it requiresno coherent detection. It is accomplished by controlling thetransmission of data by modulating the intensity of the light for theoptical signal using a radio-frequency (RF) carrier. Since there is nomodulation of the phase of the light, normally also required forcoherent modulation, INBM greatly reduces the complexity, form factorand power usage of the transmitter and/or receiver by eliminating theneed for components associated with coherent detection/modulation.

Referring to FIG. 7, an illustrative flowchart illustrating operationsof the INBM-based transmitter is shown. Herein, a dispersive fiberchannel can be viewed as a collection of contiguous sub-channels (orfrequency bands) separate by channel nulls. The number of frequencybands (R) is dependent on the number of channel nulls. INBM separatesthe data targeted for transmission in sub-blocks (block 700). Eachsub-block is to be transmitted over an available inter-null frequencybands (blocks 710, 720 and 730).

According to one embodiment, multi-level modulation techniques can beused within each band to achieve bandwidth efficient multi-bit/symboltransmission (block 740). For instance, multi-dimensional Trellis codedmodulation can be used to achieve high bandwidth efficiency while alsoproviding several decibels (dBs) of coding gain. This process isiterative for each sub-block (block 750).

FIG. 8 illustrates an embodiment of transmitter logic 800 for supportinginter-null band data transmissions. Transmitter logic 800 comprises aninter-null band (INB) multiplexer 810 and signaling logic 820 ₁-820 _(R)(R≧1), each being adapted to support a different sub-band frequencyrange between channel nulls. Signaling logic 820 ₁-820 _(R) is coupledto a radio frequency (RF) combiner 830 that creates a composite signal840. The operations of INB multiplexer 810, signaling logic 820 ₁-820_(R), and RF combiner 830 are controlled by INBM control logic 850,which may be software executed by processing circuitry that is either(i) integrated within an integrated circuit including INB multiplexer810, signaling logic 820 ₁-820 _(R), RF combiner 830 and optionally nullsearch generator 845 as represented by dashed lines, or (ii) implementedas an external control source.

More specifically, for supporting 100 gigabits per second (100 Gbps)transmissions, transmitter logic 800 receives data as ten (10) channelseach operating at 10 Gbps. According to one embodiment, the data may beprovided from a host device such as a computing device (e.g., computer,main frame, server, access point, etc.), processing circuitry or thelike.

Thereafter, the data is segmented into “R” segments of blocks by INBmultiplexer 810, where “R” denotes the number of inter-null frequencybands that can be supported for transmission. According to oneembodiment of the invention, one or more sub-blocks are allocated tocorresponding signaling logic 820 ₁-820 _(R), which are collectivelyresponsible for data transmissions over inter-null frequency bands. As aresult, it is contemplated that the number of sub-blocks routed to aparticular signaling logic 820 _(i) may vary, depending on whichinter-null channel that signaling logic 820 _(i) is assigned. The reasonis that the bandwidth of each inter-null frequency band varies dependingon its center frequency as shown in FIGS. 6A and/or 6B.

It is contemplated that transmitter logic 800 may be configured with amaximum number (R) of signaling logic 820 ₁-820 _(R), where any unusedsignaling logic components may be powered off.

According to one signaling logic 820 ₁, the allocated sub-blocks of dataare routed to an encoder 860 (e.g., M-TCM, OFDM, QAM, etc.), whichencodes the data in order to improve transmission efficiency. Theencoded data is provided to a signal conditioner 865, which is logicthat is configured to compensate for distortion caused by a driver 890and optical modulator 892 that converts data from the resultant RFcomposite signal into optical pulses for transmission over the opticalfiber medium. In other words, signal conditioner 865 pre-distorts theencoded data to compensate for distortion that will be caused bycomponents later in the optical transmission path.

The distortion-compensated, encoded data is routed to adigital-to-analog converter (DAC) 870 which converts the receiveddigital data into an analog signal having two components, an inphasecomponent (I) and a quadrature component (Q). The IQ signals areprovided as input to IQ modulator 875 for modulation of a RF carriersignal 880 which is centered at a frequency band associated with thisparticular signaling logic 820 ₁. The RF-modulated signal is passedthrough a filter 885 (e.g., bandpass filter) to ensure that RF-modulatedsignal is confined to the inter-null frequency band associated withsignaling logic 820 ₁.

RF combiner 830 receives the RF-modulated signals from differentsignaling logic 820 ₁-820 _(R) to produce composite signal 840.Composite signal 840 is provided to driver 890 which is responsible fordriving optical modulator 892 that, along with tunable laser source 895,produces optical signals to propagate data contained in RF compositesignal 840.

Upon start-up of the module (transmitter), null search generation logic845 controlled by INBM control logic 850 will be sending a succession oftones (e.g. characterization signal) that covers a frequency rangebetween a minimum frequency and a maximum frequency (e.g. 0-30 GHz) andawaits for response signaling from the receiver by sweeping thefrequency range for detecting tones and returning information as towhich tones were detected by the receiver. The detected tones are usedto represent which channels are being used for optical datatransmissions, which may be detected by receiver logic 1000 of FIG. 10.Upon any detection of a loss of signal (LOS), processing circuitrywithin transmitter logic 800 restarts the INBM control logic 850 tocompute the detected available channels using null search generationlogic 845.

It is contemplated that null search generation logic 845 is optional asnull locations can be estimated based on the distance of the opticalfiber (z) is known and the dispersion coefficient of the optical fibermedium (β₂) as set forth above in equation (1).

Referring now to FIG. 9, an illustrative embodiment of the operations oftransmitter logic 800 of FIG. 8 is shown.

First, incoming data from host is divided into R segments of blocks bythe INBM Demultiplexer, each segment including one or more blocks Bieach having a size Ki (block 900). The number of blocks in a particularseries, and block size Ki are determined by INBM control logic 850 ofFIG. 8 based on its knowledge of the null channel locations and thedesired data transmission rate (e.g., bits/second/hertz). INBM controllogic 850 may be integrated within an integrated circuit including INBMmultiplexer 810, signaling logic 820 ₁-820 _(R), RF combiner 830 andoptionally null search generator 845, as illustrated by dashed lines, ormay be implemented as an external control source.

According to one embodiment of the invention, each block Bi is processedby a multilevel Trellis-coded encoder (M-TCM) that encodes the data togenerate Inphase (I) & Quadrature (Q) signals (block 905). The M-TCMencoder allows for the transmission of J bits/symbol (J>1) therebyconserving bandwidth while still allowing for high-rate datatransmission through an increase in bandwidth efficiency(bits/second/Hertz).

In one embodiment, the M-TCM encoder could be chosen to generate of amulti-dimensional PSK or QAM constellation. Herein, multipleconstellations are combined together to produce multi-bit symbols. Forexample, in a higher-order PSK technique such as 2×8-PSK for example, atransmission rate of 3.5 bits/symbol may be achieved, thereby increasingthe bandwidth efficiency. In another embodiment, in order to furtherincrease bandwidth efficiency, two techniques (TCM, Partial Response)may be combined. “Partial Response” is a technique that is involved inthe creation of a controlled amount of inter-symbol interference intothe transmitted signal. This interference may be removed at the receiverusing Maximum Likelihood estimation. This technique, which may involvethe generation of TCM constellation followed by the application ofPartial Response may enable an increased bandwidth efficiency (e.g.,increased bits/second/symbol).

TCM is an error correction code that allows for the correction oferrors. As an ancillary benefit, the M-TCM encoder also provides for acoding gain by utilizing a (k/k+1) block or convolutional encoder tohelp improve the bit error rate.

The constellation size is determined by the INBM control logic 850 ofFIG. 8 given the bandwidth efficiency (bits/second/Hertz).

Of course, it is contemplated that the M-TCM encoder can be replaced byan OFDM modulator.

Referring still to FIG. 9, the IQ signals are further conditioned andprocessed by the signal conditioner (block 910). For example the signalcan be pre-distorted to compensate for components and/or channelnonlinearities. The output of the signal conditioner is received by adigital-to-analog converter (DAC) and the resultant analog IQ signalsfrom the DAC are used by an IQ modulator to modulate an RF carrier f_(i)(blocks 915 and 920).

The output of the modulated signals is filtered using a bandpass filtercentered at f_(i) and having a bandwidth BW_(i) (block 925). The carrierfrequency and bandpass center frequency f_(i) is selected using the INBMcontrol logic.

All “R” outputs from the bandpass filters from the signaling logiccorresponding to each of the inter-null frequency bands is combinedusing a broadband RF combiner (block 930). The output of the RF combineris applied to a linear driver (block 935). The output of the lineardriver modulates the intensity of a laser source (where tunable) usingan external optical modulator (block 940).

The optical output of the optical modulator is transmitted over anoptical medium such as an optical fiber (block 945).

FIG. 10 illustrates an embodiment of receiver logic 1000 for supportinginter-null band data transmissions. Receiver logic 1000 comprises anautomatic gain controller (AGC) 1005 that maintains the gain of theincoming optical signal constant before routing the same to an RFsplitter 1010. Furthermore, the optical signal is converted to anelectrical signal by optical receiver 1015 before routing to AGC 1005.

It is contemplated that null search generation logic 1020 is optional asnull locations can be estimated since the distance of the optical fiber(z) is known and the dispersion coefficient of the optical fiber medium(β₂) is known. Upon start-up of the module (receiver), null searchdetector logic 1020 controlled by INBM control logic 1050 is adapted todetect a succession of tones by sweeping the available bandwidth (e.g.0-30 GHz) and, for those tones that are detected, a response (not shown)is sent back to the transmitter to identify which channels are detectedby the receiver.

RF splitting 1010 receives the electrical signal, which is a compositesignal formed from RF-modulated signals, and separates each RF-modulatedsignal therefrom. The number of RF-modulated signals may be based on thenumber of channels “R” used in supporting data throughput as targeted bythe transmitter logic 800 of FIG. 8, such as ten (10) channels eachoperating at 10 Gbps. The RF-modulated signals are respectively routedto signaling logic 1025 ₁-1025 _(R).

Discussing the operations for one of signaling logic 1025 ₁-1025 _(R)(e.g., signaling logic 1025 ₁), a RF-modulated signal 1035 ₁ has beenfiltered by a corresponding tunable bandpass filter 1030 ₁ to ensurethat RF-modulated signal 1035 ₁ is confined to the inter-null frequencyband associated with signaling logic 1025 ₁. Thereafter, IQ demodulator1040 ₁ demodulates RF-modulated signal 1035 ₁ from bandpass filter 1030₁ using a local oscillator (LO) frequency 1045 ₁ set approximately to atargeted center frequency associated with this particular signalinglogic 1025 ₁. The resulting signal, a recovered analog IQ signal 1055 ₁,is provided to an analog-to-digital converter (ADC) 1060 ₁.

ADC 1060 ₁ includes an anti-aliasing filter that converts the analog IQsignal 1055 ₁ into a multi-bit digital stream that is provided toequalizer & timing recovery logic 1065 ₁, which is logic that isconfigured to compensate for distortion caused by AGC 1005 beforesupplying the data to decoder 1070 ₁. Decoder 1070 ₁ matches theencoding operation performed at transmitter 800 of FIG. 8.

Thereafter, the data that is segmented and routed over “R” data pathsvia signaling logic 1025 ₁-1025 _(R) are aggregated to provide blocks byINB multiplexer 1075, where “R” denotes the number of inter-nullfrequency bands that can be supported for transmission. According to oneembodiment of the invention, blocks received via allocated signalinglogic 1025 ₁-1025 _(R) are aggregated and sent to a host device (e.g.,processor).

Referring now to FIG. 11, an illustrative flowchart of operationsperformed by the receiver logic for supporting inter-null band datatransmissions is shown. First, an optical signal from the optical fibermedium is converted to an electrical signal using an optical receiversuch as a PIN photodiode (block 1100). The electrical output of theoptical receiver is amplified and its gain kept constant using a linearautomatic gain control block (AGC) as shown in block 1105. The output ofthe AGC is split into R signals using an RF splitter, where “R” isdetermined by the INBM algorithm (block 1110). “R” would be consistentwith the number of channels selected at the transmission stage.

Each output for the RF splitter is then applied to tunable bandpassfilter (block 1115). The output of the bandpass filter is demodulatedusing an IQ demodulator with a local oscillator “LO” (block 1120). Thebandwidth, center frequency of the bandpass filter and LO frequency aredetermined by the INBM control logic.

The output of the IQ demodulator is applied to an analog-to-digitalconverter (ADC), which includes an anti-aliasing filter and converts theinput analog signal into a multi-bit digital stream (block 1125). Thedigital output of the ADC is equalized and used to extract timinginformation using the Equalizer & Timing Recovery block (EqTR), namely asignal conditioner (block 1130). When partial-response signaling is usedin the TX, a Viterbi equalizer will be enabled in the EqTR block inorder to recover the Power Response (PR) signal.

The output of the EqTR is applied to an M-TCM decoder (matched to theM-TCM encoder of the transmission stage) as set forth in block 1135. TheOutput of the M-TCM decoder is applied to the INBM multiplexer (block1140). The INBM Mux regroups the “R” digital streams from the “R” M-TCMdecoders and sends the multiplexed data back to the host (block 1145).

It is contemplated that “R” is determined by the INBM algorithm and theM-TCM decoder can be replaced by an OFDM decoder or another type ofdecoder that matches the encoding technique at the transmitter.

The INBM function may be described as follows:

A null-search generator (NSG) produces RF tones whose frequencies rangefrom “fmin” to “fmax”. At module startup, the RF combiner output isdisable and the algorithm proceeds as follows:

Count=0 Null vector is empty For f=fmin to fmax The NSG generates RFtone T(f) which modulates the intensity of the light source Tone T(f)stays on for a pre-defined time interval The Null-search detector (NSD)detects the presence of tone T(f) as shown in the RX diagram Iftone_detect=false   Count=count+1   null(count)=f Endif; End

The Null vector is transmitted back to TX via a supervisory channel. Theinformation in the null vector includes the following:

-   -   (1) provides information about the number of channel nulls        caused by dispersion and their location;    -   (2) can be used to determine the center frequency fi and        bandwidth BWi of both TX & RX BPFs;    -   (3) dictates how many blocks Bi are needed at the TX & RX;    -   (4) Is used along with the required data rate by the M-TCM        encoder to fix constellation size;    -   (5) Once the TX/RX parameters are determined (number of blocks,        block size, center frequency and BW of BPFs, constellation size        of the M-TCM encoder), the NSG and NSD are disabled and the RF        combiner is activated.

FIG. 12 illustrates an embodiment of an optical transceiver 1200 thatcomprises transmitter logic and receiver logic. Herein, in lieu ofrelying on RF carriers for data propagation as set forth in FIGS. 6A-6B& 7-8, optical carriers are used. The spacing between optical carriersis set so that the frequency spacing between neighboring opticalcarriers is equal to twice the frequency up to the first channel null(hereinafter “the first null channel frequency”). Therefore, the numberof optical carriers is dependent on the selected data rate and fiberlength.

As an illustrative example, processing logic (e.g. ASIC 1210) of opticaltransceiver 1200 is adapted to receive data over “N” (e.g. 10) channels,each at 10 gigabits per second (Gbps), to support N×10 Gbpstransmissions. According to one embodiment, the data may be providedfrom a host device such as a computing device (e.g., router, computer,main frame, server, access point, etc.), processing circuitry or thelike. Processing logic 1210 performs signal processing on the receiveddata. Thereafter, the data is provided to transmitter logic thatcomprises one or more laser drivers 1220 along with one or morecorresponding tunable laser sources and optical modulators(“laser/modulator”) 1230.

Herein, for “p” laser/modulators (p>1), each laser/modulator is tuned ata frequency spaced from each other by twice the first null channelfrequency. For instance, where a first laser/modulator 1230 ₁ isassociated with an optical carrier set to 12.5 gigahertz “GHz” (wherefirst null channel frequency is equal to 6.25 GHz), the P^(th)laser/modulator 1230 _(p) has an optical carrier set to p*12.5 GHz. Thespacing between each optical carrier is equal to twice the first nullchannel frequency.

INBM control logic 850 is adapted to control selection of multiplexer1240 and activation/deactivation of each of the laser/modulators 1240.

As further shown in FIG. 12, receiver logic comprises a demultiplexer1250 and optical detectors 1260 that are controlled by INBM controllogic 850 as well. Each of the optical detectors 1260 is associated withan optical carrier and these optical carriers are spaced from each otherby twice the first null channel frequency as well.

While the invention has been described in terms of several embodiments,the invention should not be limited to only those embodiments described,but can be practiced with modification and alteration within the spiritand scope of the appended claims.

What is claimed is:
 1. A fiber optic communications system thatcomprises: a receiver having a photo-sensor that provides a receivesignal representing intensity of light received from an optical fiber;and a transmitter that transmits a channel characterization signal overthe optical fiber, said characterization signal covering a frequencyrange between a minimum frequency and a maximum frequency, wherein thereceiver determines from the receive signal a frequency position foreach null in said frequency range based, at least in part, on a lengthand a distortion coefficient of the optical fiber.
 2. The system ofclaim 1, wherein the receiver determines the frequency position for eachnull in accordance with a type of optical fiber that exceeds 200kilometers in length.
 3. The system of claim 2, wherein the transmitteremploys an orthogonal frequency division multiplexing (OFDM) modulatorwith frequency bins corresponding to said null frequency positions beingzeroed out.
 4. The system of claim 2, wherein the receiver assembles adata stream for a host by collecting data from blocks, each blockcorresponding to a channel and having a size based at least in part on abandwidth of said channel.
 5. The system of claim 4, wherein each of theparallel demodulators have independent timing recovery modules.
 6. Thesystem of claim 4, wherein each of the parallel demodulators haveindependent equalization modules.
 7. The system of claim 4, wherein eachof the parallel demodulators have independent M-TCM decoders.
 8. Thesystem of claim 2, wherein the receiver divides the receive signal usinga set of bandpass filters each having a programmable center frequencyand bandwidth that are set based on the null frequency positions,wherein the filtered receive signals are processed by a set of paralleldemodulators.
 9. The system of claim 1, wherein, based on the nullfrequency positions, the transmitter divided said frequency range intochannels and spreads data from a host into blocks, each blockcorresponding to a channel and having a size corresponding to a capacityof that channel.
 10. The system of claim 1, wherein, based on the nullfrequency positions, the transmitter divides said frequency range intochannels and spreads data from a host across a set of parallelmodulators, each modulator generating a signal for a correspondingchannel.
 11. The system of claim 10, wherein each modulator signal isfiltered by a respective bandpass filter having a programmable centerfrequency and bandwidth, said center frequency and bandwidth being setbased on the null frequency positions.
 12. The system of claim 10,wherein the transmitter includes an RF combiner that combines themodulator signals to form a transmit signal, and wherein the transmitterfurther includes an optical modulator that communicates the transmitsignal over the optical fiber.
 13. The system of claim 10, wherein theset of parallel modulators include at least one multi-dimensionalTrellis-coded modulator (M-TCM).
 14. The system of claim 13, wherein theM-TCM modulator employs a PSK or QAM signal constellation that carriesmore than 2 bits per symbol.
 15. The system of claim 13, wherein apartial-response precoder and modulator are used with the M-TCMmodulator to further increase the bits-per-symbol rate.
 16. The systemof claim 13, wherein each M-TCM modulator is coupled to a signalconditioner that at least partly compensates for expected channeleffects.
 17. The system of claim 10, wherein the set of parallelmodulators include at least one orthogonal frequency divisionmultiplexing (OFDM) modulator.
 18. The system of claim 1, wherein thereceiver determines the frequency position for each null in saidfrequency range based, at least in part, on the length and thedistortion coefficient of the optical fiber in accordance with anequation${f = {\frac{1}{2\;\pi}\sqrt{\frac{\left( {{2k} + 1} \right)\pi}{\beta_{2}z}}}},$where “z” constitutes the length and β₂ constitutes the distortioncoefficient.
 19. A fiber optic communications method comprising:dynamically identifying frequencies at which spectral nulls occur in asignal received via an optical fiber, the frequencies at which thespectral nulls occur are estimated based, at least in part, on a lengthand a distortion coefficient of the optical fiber; and segregatingcommunications over the optical fiber into a set of inter-null bandsdefined by the spectral nulls.
 20. The method of claim 19, wherein saidsegregating includes using, for each inter-null band, a respectivemodulator to generate an analog transmit signal and a respectivedemodulator to process an analog receive signal, said analog signalsbeing frequency-limited to that inter-null band.
 21. The method of claim19, further comprising setting center frequencies and bandwidths for aset of bandpass filters that conduct frequency limiting of analogtransmit signals for the inter-null bands.
 22. The method of claim 19,further comprising setting center frequencies and bandwidths for a setof bandpass filters that conduct frequency limiting of analog receivesignals for the inter-null bands.
 23. The method of claim 19, whereinthe frequencies at which the spectral nulls occur are estimated inaccordance with a type of optical fiber that exceeds 200 kilometers inlength.
 24. A method comprising: dynamically identifying frequencies atwhich spectral nulls occur in a signal received via a single-modeoptical fiber that corresponds to a long-haul optical communicationlink, the frequencies at which the spectral nulls occur are identifiedbased, at least in part, on a length and a distortion coefficient of theoptical fiber; and segregating communications over the optical fiberinto a set of inter-null bands each associated with an optical carrier,the optical carriers each being separated from each other by twice afrequency of a first channel null for a first inter-null band of the setof inter-null bands.